Both sides still think the former GOP senator from Nebraska will be confirmed, but the filibuster brought stark condemnations from Obama and Senate Democrats, who decried it as an unprecedented partisan move against a nominee to lead the Pentagon.

Mr. Brennan has played a central role in Mr. Obama's most critical and controversial counterterrorism and security policies. He has been a chief driver of Mr. Obama's acceleration of the CIA's drone program and its expansion into Yemen. He also has been central in navigating the politics surrounding the administration's inability to close the U.S. prison in Guantanamo Bay, Cuba.

Mr. Brennan also has served as an emissary to Yemeni officials to manage counterterrorism operations there and in Somalia and has helped steer white House security policy in response to the Arab Spring. [More...}